The present invention relates to a method of producing a tube-shaped arm having a buckle portion and adapted to be connected to an adaptor portion and a connectable member.
Various arms are known conventionally, including a suspension arm of an automobile and a tie rod of a steering mechanism. These arms typically have a tubular shape, with a ball joint attached to one end, and a cylindrical shaft supporting portion having a rubber bush attached to the other end. There are various conventionally known examples of a tubular-arm to which such a ball joint and a shaft supporting portion shall be connected with a given distance between the ball joint and the shaft supporting portion.
As is true with an arm disclosed in Japanese Patent Provisional Publication No. 1997-183385, these arms typically have a buckle portion for protecting the other part of the arm by buckling when receiving an external load greater than a given value.
To be more specific, an arm disclosed in Japanese Patent Provisional Publication No. 1997-183385 is a part of a rack-and-pinion steering mechanism and includes a shaft-supporting portion, a ball joint as a supporting portion, and an arm to be used as a tie rod. The shaft-supporting portion has a cylindrical portion and external threads. A rubber bush to which a rack shaft as an adaptor portion will be attached is contained in the cylindrical portion. The ball joint is adapted to be attached to a knuckle member for supporting wheels. The shaft-supporting portion and the ball joint are respectively connected to the two opposing ends of the arm.
The arm is formed in a generally cylindrical shape and has a first connecting portion at one end and a second connecting portion at the other end. Internal threads to be engaged with the external threads of the shaft-supporting portion are formed in the cylindrical inner surface of the first connecting portion. Internal threads to be engaged with the external threads of the ball joint are formed in the cylindrical inner surface of the second connecting portion. A buckle portion which is smaller in diameter and adapted to buckle when receiving a load greater than a given value is provided at the axial middle of the arm. The arm is produced by using a material having the same diameter as that of the first connecting portion and the second connecting portion. The buckle portion of the arm, which is smaller in diameter than the other part of the arm and has a hexagonal outer shape, is formed by applying pressure to the axial middle of the material from the outer surface towards the center of the material, while securing both ends of the material.
As the buckle portion of the arm disclosed in Japanese Patent Provisional Publication No. 1997-183385 is formed by applying pressure to the axial middle of the material from the outer surface, there is the possibility of parts of the first connecting portion and the second connecting portion, i.e. the parts adjacent to the buckle portion, becoming narrower together with the buckle portion, during the process of forming the buckle portion. This may prevent production of buckle portions having a given, uniform axial length, making it difficult to achieve a stable buckling strength. In other words, in order to enable the production of buckle portions having a given, uniform axial length, it is necessary to prevent undesirable deformation of the parts of the first connecting portion and the second connecting portion adjacent to the buckle portion by, for example, inserting insert dies or similar members into the first connecting portion and the second connecting portion so as to prevent reduction of the diameter of the parts of the first connecting portion and the second connecting portion adjacent to the buckle portion. This, however, may complicate the production of the arm.
As its first connecting portion and second connecting portion have nearly the same diameter, the conventional arm described above cannot be connected to a shaft supporting portion and a ball joint that are provided with external thread portions having different diameters.
In order to solve the above problem, an object of the present invention is to provide an arm production method which enables the easy and reliable production of an arm having a given, uniform buckling strength.
The present invention provides a method of producing an arm which will be connected to an adaptor portion and a connectable member and includes a tubular first connecting portion provided at one end of the arm and adapted to be connected to said adaptor portion, a tubular second connecting portion provided at the other end of the arm and adapted to be connected to said connectable member, and a buckle portion which is smaller in diameter than the first connecting portion as well as the second connecting portion and provided between the first connecting portion and the second connecting portion so as to buckle when a load greater than a given value is applied to the first connecting portion or the second connecting portion, said method calling for forming an unfinished body by narrowing one end portion of a tubular material that has a diameter corresponding to that of either one of the first connecting portion or the second connecting portion so as to form said one end portion into a small diameter portion having a diameter corresponding to that of said buckle portion, and radially enlarging the portion between the end of the slender portion of the unfinished body and the portion of the slender portion to be formed into the buckle portion so as to increase the diameter to correspond to the diameter of the other one of the first connecting portion or the second connecting portion, i.e. the connecting portion having a diameter different from that of the material. By forming an unfinished body by narrowing one end portion of a tubular material having a diameter corresponding to either one of the first connecting portion or the second connecting portion so as to form said one end portion into a small diameter portion having a diameter corresponding to that of said buckle portion, and radially enlarging the portion between the end of the slender portion of the unfinished body and the portion of the slender portion to be formed into the buckle portion so as to increase the diameter to correspond to the diameter of the other one of the first connecting portion or the second connecting portion, the invention enables the reliable formation of a buckle portion having a given length and the stable production of arms having a uniform buckling strength.
A method of producing an arm according to another feature of the invention is characterized in that the first connecting portion and the second connecting portion have different diameters and that the method calls for use of a material having a diameter that corresponds to the greater of the diameters of the first connecting portion and the second connecting portion. By using a material having a diameter that corresponds to that of the greater of the diameters of the first connecting portion and the second connecting portion, the arm is formed based on the greater of the diameters of the two connecting portions. Therefore, the invention is effective in reducing the proportion by which the diameter of the slender portion of the unfinished body is increased, thereby facilitating the production of the mold for radially enlarging the slender portion and consequently improving the manufacturability of the entire arm.
A method of producing an arm according to yet another feature of the invention is characterized in that a stepped portion having a tubular shape with a diameter greater than the buckle portion is formed between the buckle portion and at least one of the two connecting portions, i.e. the first connecting portion and the second connecting portion, by narrowing the corresponding portion or portions of the material. By the feature described above, which calls for forming a stepped portion having a tubular shape with a diameter greater than the buckle portion between the buckle portion at least one of the two connecting portions, the invention enables the formation of a buckle port that is ensured to have a given buckling strength regardless of whether the diameter of the buckle portion is greatly different from that of the first connecting portion or the second connecting portion.
A method of producing an arm according to yet another feature of the invention is characterized in that the first connecting portion and the second connecting portion have different diameters and that external threads to be engaged with either one of the adaptor portion or the connectable member are formed along the cylindrical outer surface of the connecting portion that has the smaller diameter. As external threads to be engaged with either the adaptor portion or the connectable member are formed along the cylindrical outer surface of the connecting portion that has the smaller diameter, the area of the threads to be formed is small in comparison with a structure that calls for threading the larger connecting portion. Therefore, the invention facilitates threading operation, thereby improving the manufacturability of the arm.